In the past several years, there has been a continuing interest in the development of various forms of rigid expanded thermoplastic material in the higher density ranges. Such materials in the lower density ranges have already gained wide recognition for their insulating properties for such items as coolers and for its flotation property in boats, for example. In these uses and in others often the plastic foam, as it is called, has replaced other materials such as cork and the like.
Now the more rigid and dense expandable plastics are starting to be strongly considered as substitutes for wood in a variety of uses. Such uses include the manufacturing of picture frame material and for use as decorative molding material in construction. In these forms the expandable thermoplastic material can be extruded which is now generally accepted as the most practical manufacturing method for these types of foamed plastic products.
While generally directed to all extrusions of such expandable thermoplastic material this invention is particularly directed to the manufacture of larger panels of such extruded material which have a large variety of uses. With the use of these panels for such items as doors for cabinets or in the construction of full size room doors for example, certain requirements have become evident such as the necessity of producing a smooth outer surface or skin on such panels which emulates the surface of so-called finished wood. Of course, it is most desirable to produce such a surface without any secondary operations so that it is one object of this invention to produce such a smooth outer surface or skin on the material as it comes from the extruder. To provide such a surface it will be seen that it is necessary to eliminate at the outer surface of the material the formation of a cellular structure which is typical of all plastic foam. This integrally knit cellular structure will characterize the entire internal portion of the extruded material and it is another object of this invention to be able to carefully control the kind of cellular structure produced internally especially as to the size of the cells formed. The cell size and the cross-sectional uniformity of the cells from the inside to the surface of the extruded material will determine the density of the material as well as its load bearing characteristics and its weight all of which are of course important characteristics for its intended use as a wood substitute.
Of course, the material produced may have many other uses where it is not substituting for wood at all but will be selected for its own unique characteristics.
To elaborate further on the objects of the invention; it is an object to provide both a method and apparatus for the extrusion of an elongated body of expandable thermoplastic material having substantially smooth outer surfaces and a predetermined characteristic cellular inner structure.
It is a further object of this invention to provide a method and apparatus for providing an elongated body of expanded thermoplastic material which is useable as extruded requiring no surface finishing for its intended final use.
It is a still further object of this invention to provide a method and apparatus which can produce relatively inexpensively various elongated bodies of expandable plastic material which have many uses including that of a wood substitute.